Auto Brightness should be off (as someone else mentioned) when you're doing side by side comparisons.
Sometimes there is a burn in period for LED's to reach normal operating brightness, and for adhesives to reach maximum transparency.
It is possible that there is a batch of lower quality screens, a defective unit, or screens made by a different manufacturer to a different spec.
This screen shot doesn't really give us much info though. Give the new one a week of normal usage, turn auto brightness off, then compare them again.
I'd be interested to see if there is still such a difference.